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Altimeter accuracy



 
 
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  #1  
Old May 4th 07, 01:17 AM posted to rec.aviation.owning
Jim Stewart
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Posts: 437
Default Altimeter accuracy

How close should an altimeter be to field elevation
when set to the pressure indicated by the field's
AWOS? I'm seeing a 50' error.
  #2  
Old May 4th 07, 01:37 AM posted to rec.aviation.owning
karl gruber[_1_]
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Posts: 396
Default Altimeter accuracy

75'



"Jim Stewart" wrote in message
...
How close should an altimeter be to field elevation
when set to the pressure indicated by the field's
AWOS? I'm seeing a 50' error.



  #3  
Old May 4th 07, 02:42 AM posted to rec.aviation.owning
Rip
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Posts: 75
Default Altimeter accuracy

Jim Stewart wrote:
How close should an altimeter be to field elevation
when set to the pressure indicated by the field's
AWOS? I'm seeing a 50' error.

+/- 75 feet for IFR

Rip
  #4  
Old May 4th 07, 03:39 AM posted to rec.aviation.owning
Ron Rosenfeld
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Posts: 264
Default Altimeter accuracy

On Fri, 04 May 2007 01:42:07 GMT, Rip wrote:

Jim Stewart wrote:
How close should an altimeter be to field elevation
when set to the pressure indicated by the field's
AWOS? I'm seeing a 50' error.

+/- 75 feet for IFR

Rip


It depends on your field elevation.

You should be able to check and see what the error was at the last
altimeter certification for IFR.

The tolerances, at the lower altitudes, a (Appendix E 14 CFR 43)

0 ±20'
500 ±20'
1000 ±20'
1500 ±25'
2000 ±30'
3000 ±30
4000 ±35
6000 ±40
8000 ±60

For a VFR only a/c, I do not believe there is any particular requirement.
--ron
  #5  
Old May 5th 07, 06:37 PM posted to rec.aviation.owning
Capt. Geoffrey Thorpe
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Posts: 790
Default Altimeter accuracy

"Jim Stewart" wrote in message
...
How close should an altimeter be to field elevation
when set to the pressure indicated by the field's
AWOS? I'm seeing a 50' error.


What's the accuracy of the AWOS?

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.


  #6  
Old May 7th 07, 08:53 PM posted to rec.aviation.owning
Jim Stewart
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Posts: 437
Default Altimeter accuracy

Capt. Geoffrey Thorpe wrote:
"Jim Stewart" wrote in message
...
How close should an altimeter be to field elevation
when set to the pressure indicated by the field's
AWOS? I'm seeing a 50' error.


What's the accuracy of the AWOS?


Glad you asked. I called some friends at Allweather
(which made the AWOS at my field) and they said the
units are factory calibrated to +/- 5 feet and that
FAA specifications are +/- 20 feet.




--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.


  #7  
Old May 7th 07, 09:41 PM posted to rec.aviation.owning
Jim Carter[_1_]
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Posts: 403
Default Altimeter accuracy

I think what you've cited is for bench testing during certification. Once
installed in the aircraft then comparison against the pressure as recorded
by the official weather observation is +/- 75'.

An interesting discussion point came up recently that exposed the difference
in teaching over the past 30 years. 30 years ago we were taught to set the
altimeter to the known field elevation and record the difference between the
official pressure and the indicated pressure, then apply that difference to
every setting you received along your route of flight.

Today they teach to set your altimeter to the official pressure and that's
it.

So my question becomes, when executing a precision approach to a minimum DH
of 200' and then executing the missed procedure, the aircraft is allowed to
descend slightly below the DH as things spool up. If you are already 75'
lower than you think because of altimeter error, and you descend only 20'
more (one gradient on the altimeter) aren't you really only 105' off the
deck?


--
Jim Carter
Rogers, Arkansas
"Ron Rosenfeld" wrote in message
...
On Fri, 04 May 2007 01:42:07 GMT, Rip wrote:

Jim Stewart wrote:
How close should an altimeter be to field elevation
when set to the pressure indicated by the field's
AWOS? I'm seeing a 50' error.

+/- 75 feet for IFR

Rip


It depends on your field elevation.

You should be able to check and see what the error was at the last
altimeter certification for IFR.

The tolerances, at the lower altitudes, a (Appendix E 14 CFR 43)

0 ±20'
500 ±20'
1000 ±20'
1500 ±25'
2000 ±30'
3000 ±30
4000 ±35
6000 ±40
8000 ±60

For a VFR only a/c, I do not believe there is any particular requirement.
--ron



  #8  
Old May 7th 07, 11:45 PM posted to rec.aviation.owning
Ron Rosenfeld
external usenet poster
 
Posts: 264
Default Altimeter accuracy

On Mon, 07 May 2007 20:41:13 GMT, "Jim Carter"
wrote:

I think what you've cited is for bench testing during certification. Once
installed in the aircraft then comparison against the pressure as recorded
by the official weather observation is +/- 75'.

An interesting discussion point came up recently that exposed the difference
in teaching over the past 30 years. 30 years ago we were taught to set the
altimeter to the known field elevation and record the difference between the
official pressure and the indicated pressure, then apply that difference to
every setting you received along your route of flight.

Today they teach to set your altimeter to the official pressure and that's
it.

So my question becomes, when executing a precision approach to a minimum DH
of 200' and then executing the missed procedure, the aircraft is allowed to
descend slightly below the DH as things spool up. If you are already 75'
lower than you think because of altimeter error, and you descend only 20'
more (one gradient on the altimeter) aren't you really only 105' off the
deck?


I did cite the bench testing numbers, as they are the ones of which I am
aware.

I've heard of the 75' "allowance" for a field measurement, but I've not
seen a regulatory justification for that value. As someone who flies
instrument approaches to minimums, I, personally, would not be happy with a
75' error, and would have the system rechecked.

So far as you being only 105' off the deck in your hypothetical instance,
that is obviously the case.

By the way, for Category II operations, which may have a DH as low as
100'AGL, it is a requirement to have readily available the altimeter
calibration information from the last certification, (and to apply it
appropriately).
--ron
  #9  
Old May 8th 07, 04:38 AM posted to rec.aviation.owning
Jim Carter[_1_]
external usenet poster
 
Posts: 403
Default Altimeter accuracy

Isn't a radar altimeter required equipment for CAT II and CAT III work? We
were trying to get an aircraft and aircrew certified for CAT II in Seattle
many years ago and I thought that was the reason they had the RA installed.

--
Jim Carter
Rogers, Arkansas
"Ron Rosenfeld" wrote in message
...
On Mon, 07 May 2007 20:41:13 GMT, "Jim Carter"
wrote:

I think what you've cited is for bench testing during certification. Once
installed in the aircraft then comparison against the pressure as recorded
by the official weather observation is +/- 75'.

An interesting discussion point came up recently that exposed the
difference
in teaching over the past 30 years. 30 years ago we were taught to set the
altimeter to the known field elevation and record the difference between
the
official pressure and the indicated pressure, then apply that difference
to
every setting you received along your route of flight.

Today they teach to set your altimeter to the official pressure and that's
it.

So my question becomes, when executing a precision approach to a minimum
DH
of 200' and then executing the missed procedure, the aircraft is allowed
to
descend slightly below the DH as things spool up. If you are already 75'
lower than you think because of altimeter error, and you descend only 20'
more (one gradient on the altimeter) aren't you really only 105' off the
deck?


I did cite the bench testing numbers, as they are the ones of which I am
aware.

I've heard of the 75' "allowance" for a field measurement, but I've not
seen a regulatory justification for that value. As someone who flies
instrument approaches to minimums, I, personally, would not be happy with
a
75' error, and would have the system rechecked.

So far as you being only 105' off the deck in your hypothetical instance,
that is obviously the case.

By the way, for Category II operations, which may have a DH as low as
100'AGL, it is a requirement to have readily available the altimeter
calibration information from the last certification, (and to apply it
appropriately).
--ron



  #10  
Old May 8th 07, 12:01 PM posted to rec.aviation.owning
dave
external usenet poster
 
Posts: 68
Default Altimeter accuracy

I read somewhere on the web that your A&P can adjust your altimeter by
removing a pin or a screw that allows the barometric pressure to be set
independently from the altitude. Once it's set correctly, the pin or
screw is replaced.
Dave


Jim Stewart wrote:
How close should an altimeter be to field elevation
when set to the pressure indicated by the field's
AWOS? I'm seeing a 50' error.

 




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